<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f5('+')">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<div>
    结果:<span></span>
</div>
<script>
    let i1 = document.getElementById("i1");
    let i2 = document.getElementById("i2");
    let s = document.querySelector("span");
    function f1() {
        //parseFloat 将字符串转成整数或小数
        s.innerText = parseFloat(i1.value)+parseFloat(i2.value);
        // s.innerText = i1.value*1+i2.value*1;
    }
    function f2() {
        s.innerText = i1.value-i2.value;
    }
    function f3() {
        s.innerText = i1.value*i2.value;
    }
    function f4() {
        s.innerText = i1.value/i2.value;
    }
    function f(a) {
        //获取两个文本框 给元素添加id 浏览器会自动生成以下代码
        // let i1 = document.getElementById("i1");
        // let i2 = document.getElementById("i2");
        switch (a) {
            case 1:
                s.innerText = parseFloat(i1.value)+parseFloat(i2.value);
                break;
            case 2:
                s.innerText = i1.value-i2.value;
                break;
            case 3:
                s.innerText = i1.value*i2.value;
                break;
            case 4:
                s.innerText = i1.value/i2.value;
                break;
        }
    }
    function f5(x) {
        s.innerText = eval(i1.value+x+i2.value)
    }
</script>
</body>
</html>